This is what your Sims would be seeing if you were playing now and they (somehow) got into the attic.

One of the great things of the new camera mode in Sims 2 is the ability to get right inside the house. As shown here, we can position the camera just so for a Sim's eyeview of the attic. It makes it easier to lay some carpet down. Also take this opportunity to connect the bottom floor to the attic to the roof with some stairs.

As with a basement, the game won't cover these squares automatically so do it manually. You may wish to use the bird's eye view ("top down") to help you lay carpet.

For natural light, dormers are what you need. These are basically nothing more than sticky-out pieces of roof.

For natural light in your attic, install some dormers all the way around. These are pieces of roof that essentially stick out from the main roof of your house. For internal purposes, they're essentially walls and will, space pending, take any object that fits on the wall, such as telephones, paintings and mirrors.

Dormers are installed from the roof section and come in various sizes at ludicious prices. Install them all around the roof if you want windows on all sides.

If you don't wish to make use of natural light during the daytime hours but still wish to hang a picture or two then you can build a square wall instead underneath the flat area of the roof but this method limits the space available to you.

Dormers all the way around the house.

Chop out the long length side of the dormer that's inside the attic so light can get through

You now need to chop out the long side of each dormer that's inside the attic so you end up with a series of mini bays all around. You can remove the other two panels as well if you wish but for a professional impression looking from outside, I recommend leaving them. Remove sides of dormers with the wall tool.

Install a window in each dormer, then use individual sections of wall to join each dormer to the two next to it so you have a continous wall running all around the attic. When the wall is completed with no missing sections then the attic will darken and you'll be able to see the light streaming in through the windows.

After chopping one side of the dormer off and joining the walls together, you get this.